Hubble Space Telescope Photometry Research Articles

Deep Hubble Space Telescope Photometry of Large Magellanic Cloud and Milky Way Ultrafaint Dwarfs: A Careful Look into the Magnitude–Size Relation

We present deep Hubble Space Telescope photometry of 10 targets from Treasury Program GO-14734, including six confirmed ultrafaint dwarf (UFD) galaxies, three UFD candidates, and one likely globular cluster. Six of these targets are satellites of, or have interacted with, the Large Magellanic Cloud (LMC). We determine their structural parameters using a maximum-likelihood technique. Using our newly derived half-light radius (r h ) and V-band magnitude (M V ) values in addition to literature values for other UFDs, we find that UFDs associated with the LMC do not show any systematic differences from Milky Way UFDs in the magnitude–size plane. Additionally, we convert simulated UFD properties from the literature into the M V –r h observational space to examine the abilities of current dark matter (DM) and baryonic simulations to reproduce observed UFDs. Some of these simulations adopt alternative DM models, thus allowing us to also explore whether the M V –r h plane could be used to constrain the nature of DM. We find no differences in the magnitude–size plane between UFDs simulated with cold, warm, and self-interacting DM, but note that the sample of UFDs simulated with alternative DM models is quite limited at present. As more deep, wide-field survey data become available, we will have further opportunities to discover and characterize these ultrafaint stellar systems and the greater low surface-brightness universe.

Pushing the limits of near-infrared photometry with the Gemini Multi-Conjugate Adaptive Optics System: study of crowded fields in the globular cluster M5

ABSTRACT We present the deepest J −Ks near-infrared photometry of the globular cluster M5 (NGC 5904) from observations taken with the Gemini South Adaptive Optics Imager in tandem with the Gemini Multi-conjugate adaptive optics System (GeMS) on the 8.1-m Gemini South telescope. Point spread function (PSF) photometry was carried out using a spatially variable PSF, zero-point calibrations based on correlations to a standard photometric catalogue, colour corrections, and crowding corrections. The latter corrections provided a new challenge given the field variations of the adaptive optics corrections in the central crowded regions of this cluster. The final photometric precision in our J− Ks colour–magnitude diagram exposes a dispersion among the lower main-sequence stars of M5 for the first time. This dispersion occurs below a main-sequence knee due to variations in the helium and CNO (carbon, nitrogen, and oxygen) abundances from multiple stellar populations, consistent with results from the bright evolved stars in this cluster from ultraviolet to near-infrared Hubble Space Telescope photometry and ground-based spectroscopy. This paper completes our original GeMS quality analysis programme, providing insights into adaptive optics analyses in crowded fields.

The young massive SMC cluster NGC 330 seen by MUSE

Context. The origin of the initial rotation rates of stars, and how a star’s surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Aims. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the ∼35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). Methods. We developed a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multiband Hubble Space Telescope (HST) photometry and Multi Unit Spectroscopic Explorer (MUSE) spectroscopy. This allowed us to homogeneously constrain the physical parameters of over 250 B and Be stars (i.e., B-type stars with emission lines), brighter than mF814W = 18.8 mag. Results. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B-type stars. The rotational velocities vary as a function of the star’s position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. Conclusions. The rotational velocities of the ∼35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the majority of the B-type stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different v sin i characteristics of NGC 330 compared to younger populations can be understood in this framework.

The MUSE-Faint survey

Context. Leo T (MV = −8.0) is a peculiar dwarf galaxy that stands out for being both the faintest and the least massive galaxy known to contain neutral gas and to display signs of recent star formation. It is also extremely dark-matter dominated. As a result, Leo T presents an invaluable opportunity to study the processes of gas and star formation at the limit where galaxies are found to have rejuvenating episodes of star formation. Aims. Our approach to studying Leo T involves analysing photometry and stellar spectra to identify member stars and gather information about their properties, such as line-of-sight velocities, stellar metallicities, and ages. By examining these characteristics, we aim to better understand the overall dynamics and stellar content of the galaxy and to compare the properties of its young and old stars. Methods. Our study of Leo T relies on data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, which we use to identify 58 member stars of the galaxy. In addition, we supplement this information with spectroscopic data from the literature to bring the total number of member stars analysed to 75. To further our analysis, we complement these data with Hubble Space Telescope (HST) photometry. With these combined datasets, we delve deeper into the galaxy’s stellar content and uncover new insights into its properties. Results. Our analysis reveals two distinct populations of stars in Leo T. The first population, with an age of ≲500 Myr, includes three emission-line Be stars comprising 15% of the total number of young stars. The second population of stars is much older, with ages ranging from > 5 Gyr to as high as 10 Gyr. We combine MUSE data with literature data to obtain an overall velocity dispersion of σv = 7.07−1.12+1.29 km s−1 for Leo T. When we divide the sample of stars into young and old populations, we find that they have distinct kinematics. Specifically, the young population has a velocity dispersion of 2.31−1.65+2.68 km s−1, contrasting with that of the old population, of 8.14−1.38+1.66 km s−1. The fact that the kinematics of the cold neutral gas is in good agreement with the kinematics of the young population suggests that the recent star formation in Leo T is linked with the cold neutral gas. We assess the existence of extended emission-line regions and find none to a surface brightness limit of < 1 × 10−20 erg s−1 cm−2 arcsec−2 which corresponds to an upper limit on star formation of ∼10−11 M⊙ yr−1 pc−2, implying that the star formation in Leo T has ended.

High-resolution Spectroscopic Metallicities of Milky Way Cepheid Standards and Their Impact on the Leavitt Law and the Hubble Constant

Milky Way Cepheid variables with accurate Hubble Space Telescope photometry have been established as standards for primary calibration of the cosmic distance ladder to achieve a percent-level determination of the Hubble constant (H 0). These 75 Cepheid standards are the fundamental sample for investigation of possible residual systematics in the local H 0 determination due to metallicity effects on their period–luminosity relations. We obtained new high-resolution (R ∼ 81,000), high-signal-to-noise (S/N ∼ 50–150) multiepoch spectra of 42 out of 75 Cepheid standards using the ESPaDOnS instrument at the 3.6 m Canada–France–Hawaii Telescope. Our spectroscopic metallicity measurements are in good agreement with the literature values with systematic differences up to 0.1 dex due to different metallicity scales. We homogenized and updated the spectroscopic metallicities of all 75 Milky Way Cepheid standards and derived their multiwavelength (GVIJHK s ) period–luminosity–metallicity and period–Wesenheit–metallicity relations using the latest Gaia parallaxes. The metallicity coefficients of these empirically calibrated relations exhibit large uncertainties due to low statistics and a narrow metallicity range (Δ[Fe/H] = 0.6 dex). These metallicity coefficients are up to 3 times better constrained if we include Cepheids in the Large Magellanic Cloud and range between −0.21 ± 0.07 and −0.43 ± 0.06 mag dex−1. The updated spectroscopic metallicities of these Milky Way Cepheid standards were used in the Cepheid–supernovae distance ladder formalism to determine H 0 = 72.9 ± 1.0 km s−1 Mpc−1, suggesting little variation (∼0.1 km s−1 Mpc−1) in the local H 0 measurements due to different Cepheid metallicity scales.

Comparing Globular Cluster System Properties with Host Galaxy Environment* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programs 14219 and 15265.

We present Hubble Space Telescope photometry in the optical (F475X) and near-infrared (F110W) bands of the globular cluster (GC) systems of the inner halos of a sample of 15 massive elliptical galaxies. The targets are selected from the volume-limited MASSIVE survey, and chosen to sample a range of environments from sparsely populated groups to the brightest cluster galaxies in dense clusters. We also present a quantitative model of the relation between (F475X − F110W) color and cluster metallicity [M/H], using simulated GCs. Because much of the GC population in such galaxies is built up through accretion, the metallicity distribution of the GC systems might be expected to vary with galaxy environment. The photometry is used to create a completeness-corrected metallicity distribution for each galaxy in the sample, and to fit a double Gaussian curve to each histogram in order to model the two standard red and blue subpopulations. Finally, the properties of the GC metallicity distribution are correlated against galaxy environment. We find that almost no GCS properties and host galaxy environmental properties are correlated, with the exception of a weak but consistent correlation between blue fraction and nth-nearest-neighbor surface density. The results suggest that the systemic properties of the GC systems, at least in the inner to midhalo regions, are influenced more strongly by the local environment at early times, rather than by the environmental properties we see today.

Hubble Space Telescope survey of Magellanic Cloud star clusters: UV-dim stars in young clusters

ABSTRACT Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex colour–magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in star clusters younger than ∼2 Gyr, the clusters younger than ∼800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ∼1.7-Gyr-old cluster NGC 1783 detected a group of eMSTO stars extremely dim in ultraviolet (UV) bands. Here, we use multiband Hubble Space Telescope photometry to investigate five star clusters younger than ∼200 Myr, including NGC 1805, NGC 1818, NGC 1850, and NGC 2164 in the Large Magellanic Cloud, and the Small Magellanic Cloud cluster NGC 330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC 1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a by-product, we show that the star clusters NGC 1850 and BHRT 5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.

M92 (NGC 6341) Is a Metal-complex Globular Cluster with an Atypical Primordial Population

We present a multiple stellar population study of the metal-poor globular cluster (GC) M92 (NGC 6341), which is long known for the substantial metallicity dispersion, using our own photometric system. We find two groups with slightly different mean metallicities, the metal-poor (MP) stars with [Fe/H] hk = −2.412 ± 0.03 and the metal-rich (MR) ones with −2.282 ± 0.002. The MP constitutes about 23% of the total mass with a more central concentration. Our populational tagging based on the [C/Fe] ch and [N/Fe] nh provides the mean n(P):n(I):n(E) = 32.2:31.6:36.2 (±2.4), where P, I, and E denote the primordial, intermediate, and extreme populations, respectively. Our populational number ratio is consistent with those of others. However, the MP has a significantly different populational number ratio than the mean value, and the domination of the primordial population in the MP is consistent with observations of Galactic GCs that less massive GCs contain larger fractions of the primordial population. Structural and constituent differences between the MP and MR may indicate that M92 is a merger remnant in a dwarf galaxy environment, consistent with recent suggestions that M92 is a GC in a dwarf galaxy or a remnant nucleus of the progenitor galaxy. Discrepancy between our method and those widely used for the Hubble Space Telescope photometry exists in the primordial population. Significant magnesium and oxygen depletions of −0.8 and −0.3 dex, respectively, and helium enhancement of ΔY ≳ 0.03 are required to explain the presence of this abnormal primordial group. No clear explanation is available with limited information of detailed elemental abundances.

Evidence for a bottom-light initial mass function in massive star clusters

ABSTRACT We have determined stellar mass functions of 120 Milky Way globular clusters and massive Large Magellanic Cloud/Small Magellanic Cloud star clusters based on a comparison of archival Hubble Space Telescope photometry with a large grid of direct N-body simulations. We find a strong correlation of the global mass function slopes of star clusters with both their internal relaxation times and their lifetimes. Once dynamical effects are being accounted for, the mass functions of most star clusters are compatible with an initial mass function described by a broken power-law distribution N(m) ∼ mα with break masses at 0.4 and 1.0 M⊙ and mass function slopes of αLow = −0.3 for stars with masses m < 0.4 M⊙, αHigh = −2.30 for stars with m > 1.0 M⊙, and αMed = −1.65 for intermediate-mass stars. Alternatively, a lognormal mass function with a characteristic mass log MC = −0.36 and width σC = 0.28 for low-mass stars and a power-law mass function for stars with m > 1 M⊙ also fit our data. We do not find a significant environmental dependence of the initial mass function on cluster mass, density, global velocity dispersion, or metallicity. Our results lead to a larger fraction of high-mass stars in globular clusters compared to canonical Kroupa/Chabrier mass functions, increasing the efficiency of self-enrichment in clusters and helping to alleviate the mass budget problem of multiple stellar populations in globular clusters. By comparing our results with direct N-body simulations, we finally find that only simulations in which most black holes are ejected by natal birth kicks correctly reproduce the observed correlations.

The Kormendy relation of early-type galaxies as a function of wavelength in Abell S1063, MACS J0416.1-2403, and MACS J1149.5+2223

Context. The wavelength dependence of the projection of the fundamental plane along the velocity dispersion axis, namely the Kormendy relation, is well characterised at low redshift but poorly studied at intermediate redshifts. The Kormendy relation provides information on the evolution of the population of early-type galaxies (ETGs). Therefore, by studying it, we may shed light on the assembly processes of these objects and their size evolution. As studies at different redshifts are generally conducted in different rest-frame wavebands, it is important to investigate whether the Kormendy relation is dependent on wavelength. Knowledge of such a dependence is fundamental to correctly interpreting the conclusions we might draw from these studies. Aims. We analyse the Kormendy relations of the three Hubble Frontier Fields clusters, Abell S1063 at z = 0.348, MACS J0416.1-2403 at z = 0.396, and MACS J1149.5+2223 at z = 0.542, as a function of wavelength. This is the first time the Kormendy relation of ETGs has been explored consistently over such a large range of wavelengths at intermediate redshifts. Methods. We exploit very deep Hubble Space Telescope photometry, ranging from the observed B-band to the H-band, and VLT/MUSE integral field spectroscopy. We improve the structural parameter estimation we performed in a previous work by means of a newly developed PYTHON package called MORPHOFIT. Results. With its use on cluster ETGs, we find that the Kormendy relation slopes increase smoothly with wavelength from the optical to the near-infrared (NIR) bands in all three clusters, with the intercepts becoming fainter at lower redshifts due to the passive ageing of the ETG stellar populations. The slope trend is consistent with previous findings at lower redshifts. Conclusions. The slope increase with wavelength implies that smaller ETGs are more centrally concentrated than larger ETGs in the NIR with respect to the optical regime. As different bands probe different stellar populations in galaxies, the slope increase also implies that smaller ETGs have stronger internal gradients with respect to larger ETGs.

Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II

The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with % of its stars strongly enhanced in r-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color–magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization, forming ∼80% of the stars in the galaxy, while the remainder of the stars formed ∼3 Gyr later. When the bursts are allowed to have nonzero durations, we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 ± 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence of r-process-enhanced stars demonstrates that the r-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and the r-process nucleosynthesis to be less than 500 Myr. This measurement rules out an r-process source with a delay time of several Gyr or more, such as GW170817.

The chemical compositions of multiple stellar populations in the globular cluster NGC 2808

ABSTRACT Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of ≲0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqué star.

ALMA Lensing Cluster Survey: Hubble Space Telescope and Spitzer Photometry of 33 Lensed Fields Built with CHArGE

We present a set of multiwavelength mosaics and photometric catalogs in the Atacama Large Millimeter/submillimeter Array (ALMA) lensing cluster survey fields. The catalogs were built by the reprocessing of archival data from the Complete Hubble Archive for Galaxy Evolution compilation, taken by the Hubble Space Telescope (HST) in the Reionization Lensing Cluster Survey, Cluster Lensing And Supernova survey with Hubble, and Hubble Frontier Fields. Additionally, we have reconstructed the Spitzer Infrared Array Camera 3.6 and 4.5 μm mosaics, by utilizing all the available archival IPAC Infrared Science Archive/Spitzer Heritage Archive exposures. To alleviate the effect of blending in such a crowded region, we have modeled the Spitzer photometry by convolving the HST detection image with the Spitzer point-spread function using the novel golfir software. The final catalogs contain 218,000 sources, covering a combined area of 690 arcmin2, a factor of ∼2 improvement over the currently existing photometry. A large number of detected sources is a result of reprocessing of all available and sometimes deeper exposures, in conjunction with a combined optical–near-IR detection strategy. These data will serve as an important tool in aiding the search of the submillimeter galaxies in future ALMA surveys, as well as follow-ups of the HST dark and high-z sources with JWST. Coupled with the available HST photometry, the addition of the 3.6 and 4.5 μm bands will allow us to place a better constraint on the photometric redshifts and stellar masses of these objects, thus giving us an opportunity to identify high-redshift candidates for spectroscopic follow-ups and to answer the important questions regarding the Epoch of Reionization and formation of the first galaxies. The mosaics, photometric catalogs, and the best-fit physical properties are publicly available at https://github.com/dawn-cph/alcs-clusters.

A census of OBe stars in nearby metal-poor dwarf galaxies reveals a high fraction of extreme rotators

The early Universe, together with many nearby dwarf galaxies, is deficient in heavy elements. The evolution of massive stars in such environments is thought to be affected by rotation. Extreme rotators among them tend to form decretion disks and manifest themselves as OBe stars. We use a combination of UB, Gaia, Spitzer, and Hubble Space Telescope photometry to identify the complete populations of massive OBe stars – from one hundred to thousands in number – in five nearby dwarf galaxies. This allows us to derive the galaxy-wide fraction of main sequence stars that are OBe stars (fOBe), and how it depends on absolute magnitude, mass, and metallicity (Z). We find fOBe = 0.22 in the Large Magellanic Cloud (0.5 Z⊙), increasing to fOBe = 0.31 in the Small Magellanic Cloud (0.2 Z⊙). In the thus-far unexplored metallicity regime below 0.2 Z⊙, in Holmberg I, Holmberg II, and Sextans A, we also obtain high OBe star fractions of 0.27, 0.27, and 0.27, respectively. These high OBe star fractions and the strong contribution in the stellar mass range – which dominates the production of supernovae–, shed new light on the formation channel of OBe stars, as well as on the tendency for long-duration gamma-ray bursts and superluminous supernovae to occur in metal-poor galaxies.

The whisper of a whimper of a bang: 2400 d of the Type Ia SN 2011fe reveals the decay of 55Fe

ABSTRACT We analyse new multifilter Hubble Space Telescope (HST) photometry of the normal Type Ia supernova (SN Ia) 2011fe out to ≈2400 d after maximum light, the latest observations to date of a SN Ia. We model the pseudo-bolometric light curve with a simple radioactive decay model and find energy input from both 57Co and 55Fe are needed to power the late-time luminosity. This is the first detection of 55Fe in a SN Ia. We consider potential sources of contamination such as a surviving companion star or delaying the deposition time-scale for 56Co positrons but these scenarios are ultimately disfavored. The relative isotopic abundances place direct constraints on the burning conditions experienced by the white dwarf (WD). Additionally, we place a conservative upper limit of <10−3 M⊙ on the synthesized mass of 44Ti. Only two classes of explosion models are currently consistent with all observations of SN 2011fe: (1) the delayed detonation of a low-ρc, near-MCh (1.2–1.3 M⊙) WD, or (2) a sub-MCh (1.0–1.1 M⊙) WD experiencing a thin-shell double detonation.

Cluster Cepheids with High Precision Gaia Parallaxes, Low Zero-point Uncertainties, and Hubble Space Telescope Photometry

We present Hubble Space Telescope (HST) photometry of 17 Cepheids in open clusters and their cluster mean parallaxes from Gaia EDR3. These parallaxes are more precise than those from individual Cepheids (G < 8 mag) previously used to measure the Hubble constant because they are derived from an average of >300 stars per cluster. Cluster parallaxes also have smaller systematic uncertainty because their stars lie in the range (G > 13 mag) where the Gaia parallax calibration is the most comprehensive. Cepheid photometry employed in the period–luminosity relation was measured using the same HST instrument (WFC3) and filters (F555W, F814W, F160W) as extragalactic Cepheids in Type Ia supernova hosts. We find no evidence of residual parallax offset in this magnitude range, zp = −3 ± 4 μas, consistent with the results from Lindegren et al. and most studies. The Cepheid luminosity (at P = 10 d and solar metallicity) in the HST near-infrared, Wesenheit magnitude system derived from the cluster sample is mag and −5.890 ± 0.018 mag with or without simultaneous determination of a parallax offset, respectively. These results are similar to measurements from field Cepheids, confirming the accuracy of the Gaia parallaxes over a broad range of magnitudes. The SH0ES distance ladder calibrated only from this sample gives H 0 = 72.9 ± 1.3 and H 0 = 73.3 ± 1.1 km s−1 Mpc−1 with or without offset marginalization; combined with all other anchors we find H 0 = 73.01 ± 0.99 and 73.15 ± 0.97 km s−1 Mpc−1, respectively, a 5% or 7% reduction in the uncertainty in H 0 and a ∼5.3σ Hubble tension relative to Planck+ΛCDM. It appears increasingly difficult to reconcile two of the best-measured cosmic scales, parallaxes from Gaia and the angular size of the acoustic scale of the cosmic microwave background, using the simplest form of ΛCDM to connect the two.

The Internal Line-of-Sight Kinematics of NGC 346: The Rotation of the Core Region

We present the stellar radial velocity analysis of the central of the young massive Small Magellanic Cloud star cluster NGC 346. Using VLT/MUSE integral field spectroscopy in combination with Hubble Space Telescope photometry, we extract 103 spectra of cluster member stars suited to measure accurate line-of-sight kinematics. The cluster member stars show two distinct velocity groups at and , relative to the systemic velocity of (165.5 ± 0.2) km s−1, and hint at a third group at . We show that there is neither a correlation between the velocity groups and the spatial location of the stars, nor their locus on optical color–magnitude diagrams, which makes the stellar velocity a key parameter to separate individual stellar components in such a young star cluster. Velocity group 2 shows clear rotation with Ω2 =(−0.4 ± 0.1) Myr−1, corresponding to (−4.9 ± 0.7) km s−1 at radial distance of 10 pc from the center, a possible remnant of the formation process of NGC 346 through the hierarchical collapse of the giant molecular cloud. The ionizing gas has lost any natal kinematic imprint and shows clear expansion, driven by far-ultraviolet fluxes and stellar winds of the numerous OB stars in the cluster center. The size of this expanding bubble and its expansion velocity of 7.9 km s−1 are in excellent agreement with the estimate that the latest star formation episode occurred about two million years ago.

Structural Parameters and Possible Association of the Ultra-faint Dwarfs Pegasus III and Pisces II from Deep Hubble Space Telescope Photometry

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of ( pc) and for Psc II, we measure (69 ± 8 pc) and . We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only ∼40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in an MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close (∼10–20 kpc) passage about each other just over ∼1 Gyr ago, followed by a collective passage around the LMC (∼30–60 kpc) just under ∼1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group.

Confirmation of a metallicity spread amongst first population stars in globular clusters

Stars in massive star clusters exhibit intrinsic variations in some light elements (the multiple populations phenomenon) that are difficult to explain in a fully coherent formation scenario. In recent years, high quality Hubble Space Telescope (HST) photometry has led to the characterisation of the global properties of these multiple populations in an unparalleled level of detail. In particular, the colour-(pseudo)colour diagrams known as ‘chromosome maps’ have been proven to be very efficient at separating cluster stars with a field-like metal abundance distribution (first population) from an object with distinctive light-element abundance anti-correlations (second population). The unexpected wide colour ranges covered by the first population group – traditionally considered to have a uniform chemical composition – in the chromosome maps of the majority of the investigated Galactic globular clusters have recently been attributed to intrinsic metallicity variations up to ∼0.30 dex from the study of subgiant branch stars in two metal-rich Galactic globular clusters by employing appropriate HST filter combinations. On the other hand, high-resolution spectroscopy of small samples of first populations stars in the globular clusters NGC 3201 and NGC 2808 – both displaying extended sequences of first population stars in their chromosome maps – have provided conflicting results thus far, with a spread of metal abundance detected in NGC 3201 but not in NGC 2808. We present here a new method that employs HST near-UV and optical photometry of red giant branch stars to confirm these recent results independently. Our approach was firstly validated using observational data for M 2, a globular cluster hosting a small group of first population stars with an enhanced (by ≃0.5 dex) metallicity with respect to the main component. We then applied our method to three clusters that cover a much larger metallicity range and that have well populated, extended first population sequences in their chromosome maps, namely M 92, NGC 2808, and NGC 6362. We confirm that metallicity spreads are present among first population stars in these clusters, thus solidifying the case for the existence of unexpected variations up to a factor of two of metal abundances in most globular clusters. We also confirm the complex behaviour of the mean metallicity (and metallicity range) differences between first and second population stars.

Linear Nash-Greene fluctuations on the evolution of S_8 and H_0 tensions

We present the perturbation equations in an embedded four space-time from the linear Nash-Greene fluctuations of background metric. In the context of a five-dimensional bulk, we show that the cosmological perturbations are only propagated by the gravitational tensorial field equation. In Newtonian conformal gauge, we study the matter density evolution in sub-horizon regime and on how such scale may be affected by the extrinsic curvature. We apply a joint likelihood analysis to the data by means of the Markov Chain Monte Carlo (MCMC) method for parameter estimation using a pack of recent datasets as the Pantheon Supernovae type Ia, the Baryon Acoustic Oscillations (BAO) from DR12 galaxy sample and Dark Energy Survey (DESY1). We discuss the tensions on the Hubble constant H_0 and the growth amplitude factor S_8 of the observations from Planck 2018 Cosmic Microwave Background (CMB) and the local measurements of H_0 with Hubble Space Telescope (HST) photometry and Gaia EDR3. As a result, we obtain an alleviation below sim 1sigma in the contours (S_8-varOmega _m) at 68.4% confidence level (CL) when compared with DESY1 data. On the other hand, the H_0 tension persists with sim 2.6sigma at 68.4% CL and 1.95sigma at 95.7% CL, aggravated with the inclusion of BAO data.